1. Field of the Invention
The present invention relates to a motor control method and a motor control apparatus. Particularly, the invention relates to a motor control method and a motor control apparatus having a function of detecting a state of an abnormal rotation, including runaway, of a motor that is controlled by a software servo.
2. Description of the Related Art
According to a conventional method of detecting an abnormal rotation of a motor, when a speed deviation of a speed feedback to an instructed speed exceeds a predetermined threshold value, this abnormal rotation of the motor is detected as a speed deviation excess.
The following patent document 1 has the title “Overspeed Protection Method for Motor Driven by Inverter Equipment”, and discloses a method of changing an overspeed protection range according to a speed command signal.
The following patent document 2 has the title “Method of Detecting a Runaway of a Motor and a Runaway Detecting Apparatus”, and discloses a method and an apparatus of detecting a position of a machine system driven by a servomotor, and using a position feedback to detect a runaway of the servomotor.
[Patent document 1] Japanese Patent Application Unexamined Publication No. 6-209588 (see claim 1 to claim 3, paragraph numbers [0006] to [0012] and [0018], and drawing 1 of the specification).
[Patent document 2] Japanese Patent Publication No. 3091388 (see claim 1 to claim 5, paragraph numbers [0010], [0016,] and [0023] to [0026], and drawing 1 of the specification).
However, according to these methods of detecting an abnormal rotation of a motor based on conventional techniques, when an input speed command changes stepwise, a speed deviation becomes large during acceleration or deceleration of the motor. Therefore, detection of speed deviation excess is prohibited during the acceleration or deceleration. Consequently, there is a problem that even when an abnormal rotation of a motor occurs during acceleration or deceleration, this abnormal rotation cannot be detected.
For example, according to the method described in the patent document 1, at the time of using a motor as a main spindle, an input speed command changes stepwise in many cases. Therefore, when a high speed command is input stepwise and when the motor causes a runaway in an opposite direction, the abnormal rotation of the motor cannot be detected until when a rotation speed of the motor exceeds a predetermined high value that determines an overspeed protection range. Further, an overspeed detector that protects the motor from an overspeed does not operate. Therefore, there is also a problem that a detection of the abnormal rotation of the motor becomes slow.
The abnormal-rotation state of a motor includes a state that the motor rotates at a constant rotation because the actual speed cannot reach a speed command. A position of a magnetic pole that a controller recognizes is different from the actual position of a magnetic pole of the motor. This phenomenon occurs when a torque current Iq that the controller wants to pass and a weak current Id of a magnetic flux become zero as a torque current when observed on the actual motor axis. In this case, the overspeed detector according to the method described in the patent document 1 cannot detect the abnormal rotation of the motor, because the rotation speed of the motor does not exceed the above predetermined value.
On the other hand, the method of detecting a runaway and a runaway detecting apparatus according to claim 2 use a position feedback to detect a runaway of a servomotor. Therefore, there is a problem that this method and this apparatus cannot be used for a main spindle servomotor that uses, in many cases, a speed command motor having no position feedback.